How to get stdout of a "lost" session

I don't well what's the name of the situation, I think yesterday I closed a session with a background ipython process, but I am still able to connect to it (the process is running a web server and I can load the page). Can I still get to see the stdout/stderr of the process?

If I run ps -f -Umyname I get (I deleted some unrelated lines, but the PID and PPID are the real ones):

UID        PID  PPID  C STIME TTY      STAT   TIME CMD
16131     1767  1765  0 11:20 ?        S      0:03 sshd: myname@notty
16131     1946 21019  0 11:25 ?        Sl     1:47 /usr/bin/python -m IPython.kernel -f /home/myname/.ipython/profile_default/security/kernel-c72091
16131     3946 21019  0 14:37 pts/3    Ss+    0:00 /bin/bash
16131     4003  4001  0 14:38 ?        S      0:01 sshd: myname@pts/14
16131     4004  4003  0 14:38 pts/14   Ss     0:00 -bash
16131     6366  4004  0 16:41 pts/14   R+     0:00 ps -f Umyname
16131    21019     1  0 Jul30 ?        Sl     0:52 /usr/bin/python ipython notebook 

I am sure it's not being ran inside a screen. Any idea? I am interest in PID 21019

Solution 1:

You can use strace to see the output of a running process.

$ strace -p 21019 -e write=1,2

The above command will attach to the process with PID 21019 and trace write-calls to fd 1 (stdout) and 2 (stderr).

When you're done, you can send a sigint (to strace) to detach (press ctrl + c).

edit: There are also some tools that allow you to attach a running process to a new terminal, e.g. reptyr, screenify, retty. Note that most of these use strace, gdb or similar trace-tools to accomplish this, but they tend to produce prettier output.